I. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, which comprises the step of forming a semiconductor wafer which can replace a semiconductor wafer having an epitaxial layer (to be referred to as an epitaxial-wafer hereinafter).
II. Description of the Prior Art
In recent power semiconductor devices, the resistivity of an eptiaxial-wafer must be increased in accordance with a rise in rated voltage. However, when a semiconductor substrate having a high impurity concentration is used, it is difficult to form a high resistance epitaxial layer thereon since an impurity from the semiconductor substrate diffuses into the layer. For example, it is very difficult to form an epitaxial-wafer having an n.sup.+ -n.sup.- junction with an n.sup.- -type epitaxial layer having a resistivity of 100 .OMEGA..multidot.cm or more.
In conductivity modulation type MOS FETs, an n.sup.+ -type layer and an n.sup.- -type layer must be sequentially and epitaxially grown on a p.sup.+ -type substrate. However, when such an epitaxial-wafer is formed, since compensation of impurities occurs in an n.sup.+ -p.sup.+ junction, it is difficult to obtain desired junction characteristics. Also when an impurity-doped layer having a high impurity concentration is formed not by an epitaxial growth method but by an impurity diffusion method in another impurity-doped layer having a high impurity concentration and a conductivity type opposite to that of the former, difficulties arise as described later.